This invention relates generally to microwave bandpass filters and particularly to a filter of the coupled bar type having desirable structural features.
Microwave bandpass filters are generally known. Among these are filters utilizing resonant bars which vibrate or resonate at a particular electrical frequency. Such filters may be made so that the resonant bars are disposed in the same plane in which case a combline structure results. Alternatively, the bars of alternate resonators may be on opposite spaced walls to provide an interdigital structure.
In filters of this type the resonant frequency depends principally upon the length of the resonator bar. This may be adjusted by means of tuning screws. On the other hand the bandwidth depends upon the spacing of adjacent bars and the bar width and thickness.
Particularly at the high end of the microwave range mechanical dimensions become critical and the random accumulation of mechanical tolerances may degrade the filter performance. Hence a filter made in accordance with conventional techniques may not have the desired electrical properties.
Furthermore, the resonator bars are mechanically and electrically connected to a mounting surface. Usually the bars and housing therefore are separately machined or manufactured. They must then subsequently be joined, for example, by soldering. The solder connection in turn may increase the electrical resistivity in a random manner. This, of course, degrades filter performance by introducing an additional unwanted loss.
Furthermore, such filters may have to be used in a hostile environment. This in turn may threaten the mechanical integrity of the filter because the resonator bar must be firmly secured to its mounting surface.
It has been found that microwave filters of this type particularly at the high end of the microwave frequency range are difficult to accurately produce. Furthermore, it is equally difficult to precisely achieve the desired spacing between adjacent bars which determines the bandwidth.
Concerning the general background of microwave filters and particularly of bandpass filters, reference is made to Chapter 10 of a book entitled "Microwave Filters, Impedance-Matching Networks, and Coupling Structures" by Matthaei et al. published by McGraw Hill Book Company 1964. Chapter 10 covers pages 583 to 650. Reference is particularly made to pages 614 to 631 dealing with interdigital-line filters. In this connection see particularly FIGS. 10.06-1 through 10.06-4 and FIGS. 10.07-1 to 10.07-3.
It is accordingly an object of the present invention to provide a coupled-bar microwave bandpass filter where problems due to electrical connections between the housing and the coupling bar are eliminated.
Another object of the present invention is to provide a simple and convenient construction which allows adjustment of the distances between adjacent resonant bars, facilitating experimental adjustment of filter parameters.
A further object of the present invention is to provide a bandpass filter of the type discussed that is of simple mechanical construction and which can be readily reproduced.